This invention relates to gas burner control systems and in particular to an improved gas burner control system for gas fired apparatus of the powered vent or forced draft type.
Gas fired apparatus of the powered vent type utilizes a blower connected to the exhaust vent of a combustion chamber for drawing ambient air into the combustion chamber to ensure adequate combustion of gas at the main burner and for discharging exhaust gas or combustion products from the combustion chamber to the atmosphere by way of a flue or chimney. To avoid a safety hazard in the event of a blower malfunction or a blockage in the air inlet or the flue, pressure sensitive control devices have been employed to sense the pressure variation within the exhaust vent that is produced by operation of the blower. Such control devices act to prevent or discontinue burner operation when the fluid pressure developed by the blower is below a selected value.
One form of such pressure sensitive control devices disclosed in U.S. Pat. No. 3,650,260 is a pressure sensitive electric switch which senses the fluid pressure at the inlet of the exhaust blower to control the electrical energization of a burner gas supply valve. Another form disclosed in U.S. Pat. No. 3,141,657 is a pneumatically operated valve which opens to supply gas to a burner when inlet pressure at the exhaust blower is below a selected value. Examples of other pneumatically operated valves are disclosed in U.S. Pat. Nos. 3,269,450 and 3,592,232.
The aforementioned pressure sensitive control devices are employed with other burner control apparatus which commonly includes a combination gas manifold control of the type disclosed in U.S. Pat. No. 4,009,861. Such a combination control comprises a diaphragm main valve operated between on-off positions and regulating positions by a bleed line system which controls the supply of a gas operating pressure to a diaphragm chamber with a servo regulator valve and an electromagnetically operated control valve. To prevent opening of the diaphragm main valve when the fluid pressure developed by the exhaust blower is inadequate, a pressure sensitive electrical switch is generally employed to control the application of power to the electromagnetically operated control valve. Although such pressure sensitive switches of a reliable construction are relatively costly, the alternate use of a separate pneumatically operated valve serially connected with the combination gas manifold control is even more costly and requires substantial installation space.
It also has been proposed in U.S. Pat. No. 4,334,855 to modify a conventional combination gas manifold control to incorporate a pneumatically operated flapper valve section and a gas servo pressure regulator section which are each responsive to differential pressures produced by the flow of exhaust gas through a flow restricting orifice in an exhaust stack induced by a variable-speed blower. Communication of the differential pressure to the servo regulator section is controlled by the flapper valve section which has an "on" state when the pressure differential exceeds a first predetermined value and an "off" state when the differential pressure falls below a second predetermined value. In the "on" state of the flapper valve section, gas operating pressure is supplied by the servo regulator section to the diaphragm chamber of a diaphragm main valve which regulates the rate of gas flow to a main burner in proportion to the sensed differential pressure. It will be evident that the foregoing arrangement is rather complex and expensive and therefore is not particularly suitable for low cost installations of gas fired apparatus such as heating furnaces in homes and apartment buildings.